1. Field of the Invention
The present invention relates to a doctor blade assembly, and more specifically relates to an assembly for providing uniform force on a developer roll in order to provide consistent darkness in an electrophotographic (laser) printed image across a page width.
2. Description of the Related Art
Laser printers utilize a light beam which is focused to expose a discrete portion of a photoreceptive or image transfer drum in a further attempt to attract printing toner to these discrete portions.
One component of a laser printer is the photoreceptive drum assembly. This photoreceptive drum assembly is made out of highly photoconductive material that is discharged by light photons typically embodied by a laser. Initially, the drum is given a charge by a charge roller. As the photoreceptive drum revolves, the printer shines a laser beam across the surface to discharge certain points. In this way, the laser “draws” the letters and images to be printed as a pattern of electrical charges—an electrostatic latent image. The system can also work with either a more positively charged electrostatic latent image on more negatively charged background or a more negatively charged electrostatic latent image on a more positively charge background.
The printer's laser or laser scanning assembly draws the image to be printed on the photoreceptive drum. The traditional laser scanning assembly may include a laser, a movable mirror and a lens. The laser receives the image data defined by pixels that make up the text and images one horizontal line at a time. As the beam moves across the drum, the laser emits a pulse of light for every pixel to be printed. Typically, the laser doesn't actually move the beam. Instead, the laser reflects the light beam off of a movable mirror. As the mirror moves, the light beam passes through a series of lenses. This system compensates for the image distortion caused by the varying distance between the mirror and points along the drum. The laser assembly moves in only one plane, horizontally. After each horizontal scan, the printer rotates the photoreceptor drum a preselected distance so the laser assembly can draw the next line. A print controller synchronizes this activity. The process of forming the light image on the photoreceptive drum discharges those areas where the image is formed.
When the toner becomes electrostatically charged, the toner is attracted to exposed portions of the image transfer drum. After the data image pattern is set, charged toner is supplied to the photoconductive drum. The toner is charged so that it is attracted to the areas discharged by the laser beam and repelled by the charge on the undischarged areas. Because it is oppositely charged, the toner is attracted to and clings to the discharged areas of the drum, but not to the similarly charged “background” portions of the photoconductive drum. Toner is an electrostatically charged powder with two main ingredients, pigment and plastic. The pigment provides the coloring, such as black in a monochrome printer, that forms the text and images. This pigment is blended with plastic particles, so the toner will melt when passing through the heat of a fuser assembly. The toner is stored in the toner cartridge housing, a small container built into a removable casing. The printer gathers the toner from a sump within the housing and supplies it to a developer unit or assembly using paddles and transfer rollers. The developer roll is a charged rotating roller, typically with a conductive metal shaft and a polymeric conductive coating, which receives toner from a toner adder roll positioned adjacent the developer roll. Due to the charge, the developer roll collects the more oppositively charged toner particles from the toner adder roll. A doctor blade assembly engages the developer roll to provide a consistent coating of toner along the length and surface of developer roll, by scraping or “doctoring” excess toner from the developer roll. The doctor blade may also induce a charge on the developer roll. In turn, this provides a consistent coating of toner to the photoconductive drum. When the coating of toner on the developer roll is inconsistent, too thick, too thin or bare, coating of the photoconductive drum is inconsistent and the level of darkness of the printed image may vary unintentionally, which is considered a print defect.
The electrostatic image on the photoconductive drum is charged so that the toner particles move from the developer roll onto the latent image on the photoconductive drum. With the image data toner pattern on the photoconductive drum, the drum engages a sheet of paper or media moving adjacent thereto. Before the paper moves adjacent to the drum, the medium or paper is given a negative charge by the transfer corona wire or a charged roller. This charge is stronger than the charge of the electrostatic image, so the paper can pull the toner powder away from the surface of the photoconductive drum. When a medium, printing paper, passes beneath the rotating photoconductive drum, the toner is transferred to the medium. Since it is moving at the same speed as the drum, the paper picks up the image pattern exactly. To keep the paper from clinging to the drum, it can be discharged immediately after picking up the toner.
Finally, the paper moves through the fuser assembly, which may be embodied by a pair of rollers, one or both of which can be heated. As the paper passes through these rollers, the toner melts, permanently fusing with the fibers in the paper. The paper next feeds to the output tray.
One problem with existing doctor blade assemblies is that of providing a consistent force across the axial length of the developer roll. As previously mentioned, when the coating of toner on the developer roll is not consistent, the printed image quality may suffer. Analysis of the connections of the doctor blade assembly and the developer housing has resulted in the determination that the connection provides unintended stiffening of the doctor blade in the area of these connections. A stiffer doctor blade in these areas results in less toner applied to the developer roll in that corresponding area and therefore less toner being able to be transferred to the drum in the corresponding area and subsequently a lighter image. The connections of the doctor blade assembly to the developer are located at ends of the doctor blade assembly; therefore, image darkness has suffered near ends of the blade assembly. It is preferable to have images of consistent darkness across the width of media.